The present invention relates generally to optical spectrum analysis means and more specifically to an improved imaging spectrometer using Fourier transform methods.
An imaging spectrometer acquires what is referred to as a "data cube" having two spatial and one spectral dimensions. There exist in the prior art a variety of means to this end, the applicability of each being dependent, at least in part, upon whether spectra are dispersed in zero, one, or two dimensions. Any non-imaging spectrometer may have its field of view rastered across a scene to produce a data cube, but this is a very tedious and inefficient method. The introduction of focal plane image detectors has enabled more efficient acquisition of spectrally and spatially resolved data.
Most directly, a combination of an imaging detector with a succession of filters passing various bands can also produce a data cube. Alternatively an imaging grating spectrometer disperses light in one spatial dimension, and provides imaging in an orthogonal direction. By concatenation of a succession of one dimensional slices of a scene, a two dimensional image may be reconstructed. In addition to those specifically discussed herein, there exist in the art a variety of additional methods for imaging the output of a dispersive spectrometer to produce a data cube. An imaging spectrometer base on a Fizean interferometer also exists in the prior art, which spectrometer is based on the Fourier transform of a spatially encoded interferogram in one dimension, with the orthogonal dimension providing a one dimensional spatial image. As with imaging grating spectrometers, the imaging Fizean spectrometer builds up the second spatial dimension of a data cube by concatenation of a succession of one dimensional slices. Fourier transform spectrometers have become a preferred means for measuring spectra in a variety of applications. A book entitled Remote Sensing by Fourier Transform Spectrometry, written by Reinhard Beer and published in 1992 by John Wiley and Sons, Inc., details much of the known state of the art in this science (particularly as the science is applied to the field of atmospheric spectrometry). The first two chapters of the above referenced work are relevant to the present invention. Chapter one addresses the basic principles of Fourier Transform Spectrometry and Chapter Two addresses the characteristics of an "ideal" Fourier transform spectrometer. Briefly, a Fourier transform spectrometer is an adaption of a conventional Michelson interferometer which produces an indication of spectra (an "interferogram") the Fourier transform of which yields the spectrum. A well known technique known as fast Fourier transform ("FFT") is generally employed as the most expedient means to this end. An imaging Fourier transform spectrometer can be produced by recording multiple images at the detector focal plane of a conventional Fourier transform spectrometer, instead of the single point measurements of current instruments. A detector array is employed to detect the series of images. The variation of light intensity observed at any given pixel then constitutes the interferogram for that pixel, and the Fourier transform algorithm then yields the corresponding spectrum for that pixel.
Although a variety of imaging spectrometer systems exist in the prior art, to the inventor's knowledge none has had throughput capabilities sufficient such that it is possible to produce a real time display of a scene as viewed with a digital filter, thus using the digital filter to produce a spectral "fingerprint" for selective imaging of just those areas in the field of view which correspond to material possessing the specified spectral signature. To the inventor's knowledge, no means has previously accomplished the real time acquisition and display of high spectral resolution imaged data.